Transparent conductive films have high electrical conductivities and high transmittances in the visible light region. Further, transparent conductive films are utilized as electrodes and the like of solar cells and liquid crystal display devices as well as various kinds of light receiving elements, making use of the above-described properties thereof. Furthermore, transparent conductive films are also utilized as heat ray reflection films for use in window glass and the like of automobiles and buildings, various kinds of antistatic films, and transparent heating members for preventing fog for freezer showcases and the like, making use of reflection and absorption properties thereof in the near-infrared region.
Moreover, in the transparent conductive films, tin oxide (SnO2) containing antimony or fluorine as a dopant, zinc oxide (ZnO) containing aluminum, gallium, indium, or tin as a dopant, indium oxide (In2O3) containing tin, tungsten, or titanium as a dopant, and the like are generally broadly utilized. In particular, indium oxide films containing tin as a dopant, i.e., In2O3—Sn-based films, are called ITO (Indium tin oxide) films, and have been widely industrially used heretofore because low-resistance transparent conductive films can be easily obtained.
Further, as processes for forming such transparent conductive films, vacuum vapor deposition processes, sputtering processes, processes in which a coating fluid for forming a transparent conductive layer is applied, and the like are generally used. Of these processes, vacuum vapor deposition and sputtering processes are effective approaches for the case where a material having a low vapor pressure is used or where precise film thickness control is needed, and are industrially useful because operation is very easy. Moreover, in comparison between vacuum vapor deposition and sputtering processes, vacuum vapor deposition processes can form films faster, and are therefore superior in terms of mass productivity.
In general, vacuum vapor deposition is a process in which a solid or liquid as an evaporation source is once decomposed into gas molecules or atoms by heating in a vacuum of approximately 10−3 to 10−2 Pa, and then the gas molecules or atoms are condensed on a surface of a substrate as a thin film again. Moreover, methods for heating the evaporation source generally include the resistance heating method (RH methods) and the electron beam heating method (EB method, electron beam deposition method), and also include methods using a laser beam, the high frequency induction heating method, and the like. Further, there have also been known flash deposition, arc plasma deposition, reactive deposition, and the like, which fall into the category of vacuum vapor deposition.
In the case where a film of an oxide such as the above-described ITO is deposited, the above-described electron beam deposition has been historically often utilized. Specifically, by using an oxide evaporation material (also referred to as ITO tablet or ITO pellet) of ITO as an evaporation source, introducing O2 gas as a reactant gas into a deposition chamber (chamber), and accelerating thermoelectrons exiting from a filament (usually, a W wire) for generating thermoelectrons using an electric field to irradiate the oxide evaporation material of ITO with the thermoelectrons, the irradiated portion is locally heated to a high temperature to evaporate and be deposited on a substrate. Moreover, activated reactive evaporation (ARE) is also a useful method for ITO deposition. In the activated reactive evaporation, a plasma is produced using a thermoelectron emitter or a radio frequency discharge, an evaporant and a reactant gas (O2 gas or the like) are activated by the plasma, and thus a low-resistance film can be formed on a substrate at a low temperature. Further, recently, high-density plasma-assist evaporation (HDPE) using a plasma gun has proved to be an effective approach for ITO deposition, and has started being industrially broadly used (see Non-Patent Document 1: “Vacuum,” Vol. 44, No. 4, 2001, pp. 435-439). In this method, an arc discharge produced using a plasma generating device (plasma gun) is utilized. The arc discharge is maintained between a cathode incorporated in the plasma gun and a crucible (anode) for an evaporation source. Electrons emitted from the cathode are guided by a magnetic field and applied to part of an oxide evaporation material of ITO placed in the crucible in a focused manner. An evaporant evaporates from a portion locally heated to a high temperature by being irradiated with this electron beam, and is deposited on a substrate. Since the vaporized evaporant and the introduced O2 gas are activated in this plasma, an ITO film having favorable electrical properties can be formed. Moreover, in another method of classifying these various vacuum vapor deposition methods, vacuum vapor deposition methods involving the ionization of an evaporant and a reactant gas are collectively called ion plating (IP method), and are effective as methods for obtaining an ITO film having a low resistance and a high transmittance (see Non-Patent Document 2: “Technology of transparent conductive film,” Ohmsha, Ltd., 1999, pp. 205-211).
Further, in any type of solar cells to which the above-described transparent conductive film is applied, the above-described transparent conductive film is necessary for an electrode on a front side on which light is incident. Heretofore, the above-described ITO film, an aluminum-doped zinc oxide (AZO) film, or a gallium-doped zinc oxide (GZO) film has been utilized. Further, these transparent conductive films are required to have properties such as a low resistance and a high transmittance for visible light. Moreover, as methods for forming these transparent conductive films, vacuum vapor deposition methods such as the aforementioned ion plating and high-density plasma-assist evaporation are used.
The above-described ITO, AZO, and GZO films are materials having low resistances and high transmittances in the visible region, but have low transmittances in the near-infrared region. This is because these materials have high carrier concentrations, and hence near-infrared light is absorbed or reflected. However, in recent years, a high-efficiency solar cell has been being urgently developed in which the energy of near-infrared light is also effectively utilized by using a transparent conductive film having a high transmittance in the visible to near-infrared region and a high electrical conductivity as an electrode on a front side. Further, as such a transparent conductive film, a crystalline transparent conductive film (crystalline In—W—O) made of tungsten-containing indium oxide is disclosed in Patent Document 1 (Japanese Patent Application Publication No. 2004-43851). Moreover, the inventors of the present invention have revealed that a crystalline transparent conductive film made of cerium-containing indium oxide also has features similar to those of the above-described crystalline In—W—O film, and have found that the crystalline transparent conductive film made of cerium-containing indium oxide exerts more excellent transparency in the near-infrared region and electrical conductivity.
On the other hand, the above-described thin films containing indium oxide, tin oxide, or zinc oxide as a main component are also utilized as optical films. These thin films are high-refractive-index materials with refractive indices of 1.9 to 2.1 in the visible region, and enable the effect of interference of light to be exerted when the thin films are combined with low-refractive-index films with refractive indices of 1.3 to 1.5 in the visible region, such as silicon oxide films and metal fluoride films, to form stacked bodies. Specifically, by precisely controlling the thicknesses of films of a stacked body, antireflection effect or reflection enhancement effect in a specific wavelength region can be imparted to the stacked body. In the case of this application, a high-refractive-index film with a higher refractive index is more useful because a strong interference effect can be more easily obtained.
Further, Patent Document 2 (Japanese Patent Application Publication No. 2005-242264) shows that a cerium-containing indium oxide film has a higher refractive index than the above-described tin oxide films, zinc oxide films, and the like, and discloses an example in which the cerium-containing indium oxide film is utilized as an optical film. Furthermore, Patent Document 3 (Japanese Patent No. 3445891) and Patent Document 4 (Japanese Patent Application Publication No. 2005-290458) disclose techniques relating to a sputtering target material (In—Ce—O) of cerium-containing indium oxide and a transparent conductive film obtained by sputtering using this sputtering target material. Specifically, Patent Document 3 discloses that a transparent conductive film with high transparency and excellent heat resistance can be achieved by stacking a cerium-containing indium oxide-based transparent conductive film and a Ag-based ultra thin film on top of each other, because a cerium-containing indium oxide-based transparent conductive film has poor reactivity with Ag. Meanwhile, Patent Document 4 discloses that a film with excellent etchability can be obtained.
In the case where a thin film such as the above-described transparent conductive film or optical film is formed by a vacuum vapor deposition method such as electron beam deposition, ion plating, or high-density plasma-assist evaporation, a sintered body having a small size (e.g., a circular cylindrical shape having a diameter of approximately 10 to 50 mm and a height of approximately 10 to 50 mm) is used as an oxide evaporation material in this vacuum vapor deposition method. Thus, the quantity of films capable of being formed using one oxide evaporation material is limited. Further, when the consumption of the oxide evaporation material becomes large and the residual amount thereof becomes small, it is necessary to suspend deposition, introduce atmospheric air into a deposition chamber in a vacuum, replace the oxide evaporation material with an unused oxide evaporation material, and evacuate the deposition chamber to a vacuum again. This has been a factor in the deterioration of productivity.
Moreover, as a technique necessary for mass-producing thin films such as transparent conductive films or optical films by a vacuum vapor deposition method such as electron beam deposition, ion plating, or high-density plasma-assist evaporation, there are methods in which the above-described oxide evaporation materials are continuously fed. One example of the methods is described in Non-Patent Document 1. In this continuous feed method, circular cylindrical oxide evaporation materials are housed in a row in a cylindrical hearth, and the oxide evaporation materials are sequentially pushed out to be continuously fed with a sublimation surface maintained at a constant height. Further, the oxide evaporation material continuous feed method enables the mass production of thin films such as transparent conductive films and optical films by vacuum vapor deposition.
Cerium-containing indium oxide films are generally formed by sputtering as disclosed in Patent Documents 3 and 4. However, in recent years, there has been a strong demand for formation by various vacuum vapor deposition methods which are advantageous in terms of productivity.
However, there have been few techniques relating to oxide evaporation materials for stably forming cerium-containing indium oxide films by vacuum vapor deposition. Accordingly, techniques for making sintered bodies as sputtering targets have been adopted so far to manufacture the oxide evaporation materials.
It should be noted, however, that in a method in which a technique relating to a sputtering target is adopted, a sintered body after firing has a surface chemical composition different from the chemical composition of the interior thereof, and is therefore shaped into a tablet (oxide evaporation material) having a predetermined shape by removing the surface thereof by grinding. This makes it possible to obtain a tablet having a uniform chemical composition from the surface thereof to the interior thereof, but there have been problems such as high manufacturing cost. Moreover, in the method using the adopted technique, the density of the sintered body obtained is high, and shrinkage during sintering is large. Thus, there has also been the problem that desired dimensions are difficult to obtain after sintering. Accordingly, due to the problems of the deviation of the surface chemical composition of the sintered body and shrinkage during sintering, a slightly large sintered body is prepared in advance, and the surface thereof is removed by grinding, thus obtaining a sintered body without chemical composition deviation having desired dimensions. However, since the density of the sintered body obtained is high in the first place, there have been problems such as the cracking of the tablet due to thermal stress during vapor deposition.
On the other hand, a predetermined shape can be obtained without performing the above-described grinding or the like after firing by carrying out a sintering method in which consideration is given to the percentage of shrinkage during sintering in advance. For example, employment of a method for making an ITO tablet enables a tablet (oxide evaporation material) having desired dimensions to be obtained without performing grinding after firing. However, a cerium-containing indium oxide sintered body made by such a method also has different chemical compositions between the surface and interior thereof. A reason for this is as follows: since a cerium-containing indium oxide sintered body include a mixture of two separate phases, i.e., a crystalline phase of a solid solution of cerium in indium oxide and a crystalline phase of cerium oxide, the indium oxide phase having a high vapor pressure more easily evaporates at the surface of the sintered body at a high temperature during the production of the sintered body. On the other hand, the aforementioned sintered body of ITO includes a crystalline phase of a solid solution of tin in indium oxide and a crystalline phase of tin indate compound with no tin oxide phase remaining, and is therefore less prone to the above-described problem. Further, when deposition is performed using an oxide evaporation material obtained from a sintered body having different chemical compositions between the surface thereof and the interior thereof, the chemical compositions of thin films greatly fluctuate in early stages of the deposition. Thus, film portions formed in the early stages cannot be used. Accordingly, there have been problems such as a low production amount of thin films per tablet.
The present invention has been made by focusing attention on such problems. An object of the present invention is to provide an oxide tablet for vapor deposition (oxide evaporation material) containing indium oxide as a main component and cerium and having a uniform chemical composition from the surface thereof to the interior thereof.